A high-voltage bushing is a component that can be used to carry current at high potential from an encapsulated active part of a first high-voltage component, such as a transformer, a generator or a circuit breaker, through a barrier, for example, a grounded housing of the first component, to a second high-voltage component, for example, a high voltage line. Such a bushing can be used for example, in switchgear installations or in a high-voltage machine, like a generator or a transformer, for voltages up to several hundred kV, for example for voltages between 24 and 800 kV. In order to decrease and control the electric field, the bushing can include a condenser core which facilitates electrical stress control through at least one electrically conductive or semiconductive field grading element, which in an electrically insulating manner, can be embedded in an insulator of the condenser core. The condenser core decreases the electric field gradient and distributes the electric field homogeneously along the length of the bushing.
Such a condenser core can be manufactured as described herein. A layered spacer is wound around a central conductor, a tube or a mandrel from an electrically insulating substance and field grading elements which are arranged between successive layers of the spacer. Such a structure is called a prefabricated condenser core. The prefabricated condenser core is put in a mold and impregnated with a liquid resin, for example, an epoxy. Subsequent curing of the resin forms the condenser core in which the prefabricated condenser core is embedded in an electrically insulating matrix material of the cured resin.
Production of cast epoxy condenser core parts can be made in a vacuum molding process or in an APG (Automated pressure gelation) process.
In the vacuum molding process, molds are preheated to an appropriate casting temperature and then put in an autoclave where the epoxy resin is cast under vacuum. Once the molds are filled, they are brought to ovens where they are heated to the appropriate curing temperature. After curing, the molds cool down and are opened. The work pieces are taken out and the molds cleaned and reassembled for the next casting cycle.
In an APG Process the mold can be split in half with a mold split line along an axis of the condenser core. The two mold halves are fixed on heated plates of a horizontal press. These plates have different heating zones to create a temperature gradient that forces a curing front in one direction to allow the compensation of the shrinkage. To compensate for shrinkage, the liquid material is put under pressure from the side that cures last. The mold in the APG process can also be surrounded by a vacuum frame if impregnation takes place under vacuum. The vacuum frame is particularly useful when the mold is assembled of a larger number of parts with complicated partition surfaces running between them.
Such a mold can include two elongated halves and at its ends can be very difficult to be appropriately sealed, due to very long partition lines axially extending along the condenser core and due to triple points on places where the partition lines between the two halves of the mold meet the partition lines between the halves and the condenser core. In consequence, leakages of the resin can appear when the pressure is increased in the compensation vessel to generate resin flow for appropriately compensating the molding shrinkage.
A mold and a device for forming a high voltage insulator are disclosed in U.S. Pat. No. 6,019,931.